1. Field of the Invention
The present invention generally relates to optical measurements, and more particularly, to a method and apparatus for detection of an endpoint in the fabrication of integrated circuits.
2. Discussion of the Related Art
Modern integrated circuit devices are complex structures formed by many repeated steps of material deposition, reaction, modification (e.g., annealing), and removal. These processes are common to the fabrication of integrated circuit devices. While typical dimensions of lateral features in integrated circuit devices are frequently in the range of several microns, the thickness of fabrication layers is often far smaller than this scale, sometimes down to the range of 10 nanometers. A thickness of 10 nm corresponds to approximately 50 atomic layers. It can be easily understood that precise measurement of the dimensions during processing is crucial to device performance and device yield in the manufacturing process.
One of the critical general issues during the manufacture of an integrated circuit device is the detection of a boundary during the removal of material. Material removal may occur by methods such as chemical etching or by a plasma process such as reactive ion etching. It is generally necessary to determine the time at which all of the desired material has been removed and for which the underlying layer has not been etched significantly. This determination generally constitutes detection of an end point.
In the removal of material, the etched material and the underlying material are frequently made of different compositions, such as silicon dioxide on a silicon layer. They may, however, differ primarily in their crystalline structure or doping level and not in their gross chemical composition. This poses particular challenges for end point detection during an etching process. For example, in the manufacture of bipolar transistors, the fabrication step of forming an emitter opening involves etching an area of a deposited layer of polycrystalline silicon down to an epitaxial (single crystal) layer. This operation may be performed by a wet chemical etching bath which does not present any obvious means for real-time etching end point detection.
The consequences of improper etching are, however, significant. Underetching will cause a degradation of the gain of the transistor, while over etching will result in a degraded contact between the intrinsic and extrinsic base regions. End point detection of etching is therefore important and critical for preventing over etching and for control of undercutting of stacked layers used in VLSI processing. Furthermore, end point detection can also prevent sidewall erosion due to unnecessary over etching.
While some etching processes may provide selective removal of the overlayer and a slow rate of removal of the underlying material, this is not always attainable in practice. A real manufacturing process has numerous constraints, such as obtaining high etch rates and using relatively safe chemicals. The manufacturing process step may also require a strongly anisotropic etch rate. These conditions collectively mean that a highly selective etch may not be available. In this later instance, the capability of monitoring the etching process during manufacturing of an integrated circuit device becomes of critical importance.
An etch end point must be accurately predicted and/or detected to terminate etching abruptly. Etch rates, etch times, and etch end points are difficult to consistently predict due to lot-to-lot variations in film thickness and constitution, as well as etch bath temperature, flow, and concentration variability. That is, an etch rate is dependent upon a number of factors, which include, etchant concentration, etchant temperature, film thickness, and the film characteristics. Precise control of any of these latter factors, for example, concentration control, can be very expensive to implement and thus be cost prohibitive.
Currently, most etch rate end point determination techniques depend on indirect measurement and estimation techniques. Some etch monitoring techniques rely on external measurements of film thickness followed by etch rate estimation and an extrapolated etch end point prediction. However, etch rate may vary due to batch-to-batch differences in the chemical and physical characteristics of the film or the etchant or for a variety of other reasons as outlined above. These extrapolation methods are simply inadequate.
Real-time, in-situ monitoring of an end point is preferred. Some in-situ techniques monitor the etch rate of a reference thin film. This may require additional preparation of a monitor wafer containing the reference thin film or a suitable reference may be unavailable. Still other techniques require physical contact of electrical leads with the wafer being etched and electrical isolation of those leads and associated areas of the wafer from the etchant. This presents problems associated with contamination, contact reliability and reproducibility, and physical constraints which affect ease of use in the manufacturing process or automation of the manufacturing process.
It would thus be desirable to provide a method and apparatus which provides real-time, in-situ monitoring of an etching end point of a film on the surface of a wafer being etched.